Plasmonic Transducer Having Two Metal Elements with a Gap Disposed Therebetween

ABSTRACT

A plasmonic transducer includes at least two metal elements with a gap therebetween. The metal elements are elongated along a plasmon-enhanced, near-field radiation delivery axis. Cross sections of the metal elements in a plane normal to the delivery axis vary in shape along the delivery axis. A waveguide is disposed along an elongated side of the plasmonic transducer. The waveguide is optically coupled to the plasmonic transducer along the elongated side.

SUMMARY

Various embodiments described herein are generally directed to anear-field transducers that may be used, e.g., for heat assistedmagnetic recording. In one embodiment, a plasmonic transducer includesat least two metal elements with a gap therebetween. The metal elementsare elongated along a plasmon-enhanced, near-field radiation deliveryaxis. Cross sections of the metal elements in a plane normal to thedelivery axis vary in shape along the delivery axis. A waveguide isdisposed along an elongated side of the plasmonic transducer. Thewaveguide is optically coupled to the plasmonic transducer along theelongated side.

In another embodiment, a method involves delivering light via a channelwaveguide to an elongated portion of a plasmonic transducer thatincludes at least two metal elements with a gap therebetween. The metalelements and the waveguide are coupled along a plasmon-enhanced,near-field radiation delivery axis. Cross sections of the metal elementsin a plane normal to the delivery axis vary in shape along the deliveryaxis. The method also involves providing a surface plasmon-enhanced,near-field radiation pattern proximate the output end of the plasmonictransducer in response to the receiving the light

In another embodiment, an apparatus includes a plasmonic transducer thatincludes at least two metal elements with a gap therebetween. The metalelements are elongated along a plasmon-enhanced, near-field radiationdelivery axis. Each of the elements includes at least: a tip portionproximate an output end of the plasmonic transducer and having a firstcross sectional area relative to a plane normal to the delivery axis; acoupling portion at an input end of the plasmonic transducer and havinga second cross sectional area relative to the plane that is greater thanthe first cross sectional area; and a taper portion coupled between thetip portion and the coupling portion, wherein the taper portion variesfrom the first to the second cross sectional area along the deliveryaxis.

These and other features and aspects of various embodiments may beunderstood in view of the following detailed discussion and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The discussion below makes reference to the following figures, whereinthe same reference number may be used to identify the similar/samecomponent in multiple figures.

FIG. 1 is a perspective view of a near-field transducer and dielectricchannel waveguide according to an example embodiment;

FIGS. 2A and 2B are front and top views of the near-field transducer andwaveguide shown in FIG. 1;

FIG. 3A is a graph showing optical power transfer as a functiontransducer coupling length according to an example embodiment;

FIGS. 3B, 4A and 4B are graphs showing numerical modeling results ofoptical power absorbed in a recording layer as a function of near-fieldtransducer dimensions according to example embodiments;

FIG. 5A is a graph illustrating a calculated light absorption profile inthe middle of a recording layer for a near-field transducer according toan example embodiment;

FIGS. 5B and 6A are graphs showing numerical modeling results of opticalpower absorbed in a recording layer as a function of near-fieldtransducer dimensions according to additional example embodiments;

FIGS. 6B and 7A are graphs showing respective energy absorption profilesand temperature rise in the middle of a recording layer according toadditional example embodiments;

FIG. 7B is a cross sectional view of a near filed transducer andwaveguide according to another example embodiment;

FIGS. 8A-8B, 9A-9B, and 10 are graphs showing numerical modeling resultsof optical power absorbed in a recording layer as a function ofnear-field transducer dimensions according to additional exampleembodiments;

FIGS. 11A-11E and 12A-12H are respective perspective and cross sectionalviews illustrating a manufacturing process used to form a near fieldtransducer and waveguide according to example embodiments;

FIGS. 13A-13F are perspective views illustrating manufacturing processesused to form a near field transducer and waveguide according to anotherexample embodiment;

FIG. 14 is a perspective view of a thermal assisted recording sliderutilizing a near field transducer and waveguide according to an exampleembodiment;

FIGS. 15A-15B are cross sectional views of a near-field transducer andwaveguide with proximate recording pole according to exampleembodiments;

FIG. 16 is a flowchart illustrating a manufacturing process used to forma near field transducer and waveguide according to example embodiments;and

FIG. 17 is a flowchart illustrating a manufacturing process used to forma near field transducer and waveguide according to another exampleembodiment.

DETAILED DESCRIPTION

The present disclosure relates to a gap-plasmon, near-field transducer(NFT) that is generally formed by tapering a slot waveguide. The NFT mayinclude three sections. The first section excites the gap-plasmon byevanescent coupling from a dielectric channel waveguide. The secondsection tapers the gap-plasmon waveguide to achieve a desired opticalspot size. The third section facilitates impedance matching to couplelight into a storage medium. An NFT of this configuration may be usablein applications such as heat-assisted magnetic recording (HAMR), alsosometimes referred to as thermal-assisted magnetic recording.

It will be appreciated that the NFT and waveguide described herein maybe usable in any situation where a beam of highly focused and relativelypowerful electromagnetic energy is desired. As mentioned above, one suchapplication is in thermal/heat assisted magnetic recording, referred toas HAMR. In reference now to FIG. 14, a perspective view shows anexample HAMR slider 1400. This example slider 1400 includes anedge-emitting laser diode 1402 integrated into a trailing edge surface1404 of the slider 1400. The laser diode 1402 is proximate to a HAMRread/write head 1406, which has one edge on an air bearing surface 1408of the slider 1400. The air bearing surface 1408 faces and is heldproximate to a moving media surface (not shown) during device operation.

The laser diode 1402 provides electromagnetic energy to heat the mediasurface at a point near to the read/write head 1406. Optical couplingcomponents, such as a waveguide 1410, are formed integrally within theslider device 1400 to deliver light from the laser 1402 to the media. Inparticular, a local waveguide and NFT 1412 may be located proximate theread/write head 1406 to provide local heating of the media during writeoperations. While the laser diode 1402 in this example is an integral,edge firing device, it will be appreciated that the waveguide/NFT 1412may be applicable to any light source and light delivery mechanisms. Forexample, surface emitting lasers (SEL) may be used instead of edgefiring lasers, and the slider may use any combination of integrated andexternal lasers.

A HAMR device utilizes the types of optical devices described above toheat a magnetic recording media (e.g., hard disk) in order to overcomesuperparamagnetic effects that limit the areal data density of typicalmagnetic media. In order to record on this media, a small portion of themedia is locally heated while being written to by a magnetic write head.A coherent light source such as a laser may provide the energy to createthese hot spots, and optical components, e.g., built in to a slider thathouses the write head, are configured direct this energy onto the media.

When applying light to a HAMR medium, the light is concentrated into asmall hotspot over the track where writing takes place. To create thissmall hot spot, energy from a light source (such as a laser that isintegral to or separate from the write head) may be launched into awaveguide integrated into a hard drive head. The light propagatesthrough the waveguide and may be coupled to an optical NFT, e.g., eitherdirectly from the waveguide or by way of a focusing element.

The NFT may be located at an air bearing surface (ABS) of a slider, andmay be placed in close proximity to a write head that is also part ofthe slider. This co-location of the NFT with the write head facilitatesheating the hot spot during write operations. The waveguide and NFT maybe formed as an integral part of the slider that houses the write head.Other optical elements, such as couplers, mirrors, prisms, etc., mayalso be formed integral to the slider. The optical elements used in HAMRrecording heads are generally referred to as integrated optics devices.

The field of integrated optics relates to the construction of opticsdevices on substrates, sometimes in combination with electroniccomponents, to produce functional systems or subsystems. For example, anintegrated optics device may transfer light between components viarectangular dielectric slab or channel waveguides that are built up on asubstrate using layer deposition techniques. These waveguides may beformed as a layer of materials with appropriate relative refractiveindices so that light propagates through the waveguide in a similarfashion as through an optic fiber.

As a result of what is known as the diffraction limit, opticalcomponents cannot be used to focus light to a dimension that is lessthan about half the wavelength of the light. The lasers used in someHAMR designs produce light with wavelengths on the order of 800-900 nm,yet the desired hot spot is on the order of 50 nm or less. Thus thedesired hot spot size is well below half the wavelength of the light,and optical focusers cannot be used to obtain the desired hot spot size,due to diffraction. As a result, an NFT is employed to create thesehotspots on the media.

The NFT is a near-field optics device designed to reach local surfaceplasmon conditions at a designed wavelength. A waveguide and/or otheroptical element concentrates light on a transducer region (e.g., focalregion) near which the NFT is located. The NFT is designed to achievesurface plasmon resonance in response to this concentration of light. Atresonance, a high electric field surrounds the NFT due to the collectiveoscillations of electrons at the metal surface. Part of this field willtunnel into a storage medium and get absorbed, thereby raising thetemperature of a spot on the media as it being recorded.

In reference now to FIG. 1, a perspective view illustrates an apparatus100 having gap-plasmon NFT 102 and associated dielectric, channelwaveguide 104 according to an example embodiment. The plasmon NFT 102includes first and second metal elements 106 that may be formed of ametal such as gold (Au). The metal elements 106 are arrangedside-by-side with a gap 103 disposed therebetween. In this arrangement,the gap 103 and elements 106 can be considered to form a waveguide,herein referred to as a “slot waveguide,” to differentiate from lightdelivery, channel waveguide 104. A lower surface of the metal elements106 (e.g., surface proximate the waveguide 104) may reside on a commonplane that is parallel to a substrate plane, e.g., a plane on which thevarious components are built using wafer fabrication techniques.

The three-dimensional channel waveguide 104 includes a core 108 that maybe formed from a dielectric material such as TiOx, Ta₂O₅, ZnS, and SiNx.It will be appreciated that, within the apparatus 100, components 106,108 may be surrounded by other materials (e.g., dielectric materialssuch as alumina) that are manufactured with components 106, 108 by,e.g., using wafer fabrication techniques. For example, the waveguidecore 108 is generally surrounded by a material having a different indexof refraction, thereby acting as cladding for the waveguide 104 (see,e.g., cladding 223 in FIG. 2A). For purposes of clarity, thosesurrounding materials are not shown in FIG. 1.

In the orientation of FIG. 1, a media-facing surface 114 of theapparatus 100 (e.g., ABS) is arranged parallel to the x-z plane. An end110 of the waveguide core 108 may be disposed proximate the mediawriting surface 114, as well as respective tip portions 112 of the metalelements 106. Light is delivered from the waveguide 104 along thepositive y-direction where it is coupled to the NFT 102. The NFT 102delivers surface plasmon enhanced, near-field electromagnetic energyalong the positive y-axis (e.g., the delivery axis 115) where it exitsthe media writing surface 114. This may result in highly localized hotspot on media (not shown) when placed in close proximity to surface 114.

The metal elements 106 may include three different portions. A firstportion 116 is a directional coupler that may include elongatedbars/plates of substantially constant cross-sectional shape (at least inxz-planes along the y-direction). This portion 116 excites thegap-plasmon by evanescent coupling from the waveguide 104. A secondportion 118 tapers the gap-plasmon waveguide to achieve a desiredoptical spot size. This portion 118 may include a taper in both the x-and z-directions to form a narrower gap along the x-direction (whichcorresponds to a cross-track direction in a magnetic disk driveapparatus) and a thin film along the z-direction (down-track). The thirdportion is the aforementioned tip 112, which facilitates impedancematching between the slot waveguide 104 and a storage medium, therebyfacilitating efficient light delivery.

In reference now to FIGS. 2A and 2B, FIG. 2A shows a front (xz-plane)view of the waveguide 104 and NFT 102 of FIG. 1, and FIG. 2B shows a top(xy-plane) view of the waveguide 104 and NFT 102 of FIG. 1. For purposesof the following discussion, various dimensions are defined in FIGS. 2Aand 2B. The metal elements 106 may be assumed as symmetrical forpurposes of the present discussion, although the embodiments need not beso limited. The first portions 116 of the elements 106 have a height 202and length 204. These portions 116 are separated by a distance 206,which also defines width of the gap 103 along these portions 116. Tipportions 112 are separated by a smaller gap, 208, which may beconsidered as a continuation of waveguide gap 103. The tip portions 112have a height 210, which is generally smaller than the height 202 of thefirst portion 116.

The tapered portion 118 acts as a transition between the first portion116 and the tip portion 112. As shown here, the tapered portion includesa linear transition from respective height 202 to height 210. Similarly,the tapered portion 116 has a linear transition between the gap spacingfrom 206 to 208. While the outer width 214 near the tip is shown smallerthan width 212, this tapering may not be necessary. The widths (e.g.,width 219 seen in FIG. 2B) of tapered portions 118 along the x-directiondo not change along the y-direction, and are much larger than theskin-depth of the plasmonic material used for elements 106, such as Au,Ag. As a result, the width 214 can be chosen based on heat-dissipationrequirements without significantly impacting NFT efficiency. It will beappreciated that one or more of these transitions may use an alternatecurve/shape, e.g., rounded, parabolic, exponential, etc. Also, there isno tapered transition between the outer width 216 of tip portions 112and width 214, although one could be provided.

The waveguide core 108 is disposed below the metal portions 106,separated by a distance 218 in the z-direction. The waveguide coreitself has a height 220 and width 222. The waveguide 104, which includescore 108 and cladding 223, may extend any distance in the negativey-direction, as indicated by the broken edge 224 in FIG. 2B. In thisexample, the core 108 is shown disposed along the entire length of NFTelements 106, which includes respective lengths 204, 226, and 228 ofportions 116, 118, and 112. However, in other embodiments, the core 108need not extend over the entire NFT length, e.g., may be terminatedbefore the media surface 114. Generally, a region defined by couplinglength 204 (and possibly length 226) is generally considered to be thearea of primary coupling between the waveguide 104 and NFT 102.

The optical characteristics of apparatus 100 were modeled based on anassumed light wavelength of λ=830 nm. The dielectric channel waveguide104 was modeled as having a TiOx core 108 with Al₂O₃ cladding 223. Theindex of refraction n=2.30 for TiOx, and n=1.65 for Al₂O₃. The channelwaveguide width 222 was set to 340 nm, and height 220 was set to 300 nm.The material for the gap 103 of the slot waveguide 102 (and portionsthereinabove) was also modeled as being the same as the cladding 223,namely Al₂O₃. The metal elements 106 were modeled as Au (n=0.188+j5.39). These elements 106 act as cladding for the slot waveguide formedby the elements 106 and dielectric material in the gap 103.

A graph 300 in FIG. 3A shows optical power transfer from the TiOx core108 to the slot waveguide NFT 102 for different coupling distances 204.In the modeling the slot waveguide gap distance 206 was 300 nm, height202 was 200 nm, and spacing 218 was 50 nm. For purposes of thismodeling, the slot width 212 was assumed to be infinite. It can be seenin graph 300 that ˜60% optical power is transferred to the gap-plasmonof the slot waveguide at when length 204 is set to 2.0 μm. Note that theoptimized length 204 may be dependent on the spacing 218 between twowaveguides.

To evaluate the efficiency of the NFT 102, a storage medium was modeledas being placed proximate the media writing surface 114. The storagemedia in this model included a 12.6-nm thick Fe recording layer(n=2.94+j 3.41), a 20-nm thick MgO layer (n=1.7), and a 60-nm thick Cuheat-sink layer (n=0.26+j 5.26) on a glass substrate. The NFT-mediaspacing was 8-nm with effective index of refraction n=1.2116. The heatcapacity C (unit: J/cm3/K) and thermal conductivity K (unit: J/cm/s/K)was (C, K)=(3.14, 0.05) for the MgO layer, (3.49, 4.0) for the Cu layer,and (2.18, 0.01) for the glass substrate. It was assumed that themagnetic layer has anisotropic thermal conductivity: in-plane K=0.05,out of plane K=0.4 (C=3.62 for the magnetic layer). The results of thismodeling can be seen in FIG. 3B.

Graph 302 in FIG. 3B shows the optical power absorbed in the recordinglayer in a 50 nm by 50 nm footprint as a function of taper length 226,with tip length 228 being set to 50 nm, and tip width 216 being set to200 nm. Peak efficiency of over 30% is seen for a tip length 228 between0.8 and 1.0 μm. Graphs 400 and 402 in FIGS. 4A and 4B also show asimilar estimate of absorbed optical power as in graph 302 for differentdimensional values of the NFT 102. In graph 400, tip length 228 isvaried from 20 nm to 160 nm, with taper length 226 being set to 840 nm,and tip width 216 being set to 200 nm. In graph 400, maximum efficiencyis seen at tip length 228 of around 90 nm, or approximately 31.5 timesthe combined taper and coupling lengths 226, 204 of 2840 nm.

The peak efficiencies in FIGS. 3B, 4A, and 4B are above 30%. Laboratorytesting has found that 25-50 mW can be delivered to an NFT from a 50-100mW laser diode. To achieve a hotspot above the Curie temperature for themedia of this example, a temperature rise of around 250-300K within thehotspot is desired. As shown below (e.g., described in relation to FIG.7A below) this temperature rise can be achieved by deliveringapproximately 10 mW incident optical power to the media. As a result,the 30% efficiencies of this NFT design (and variations thereof)indicate the design is viable for HAMR applications, at least withinthese design parameters and analysis assumptions. Also note that ingraph 400, tip length 228 has a 54 nm margin at 90% efficiency. Thisrepresents achievable lapping tolerances using current manufacturingprocesses. As a result, it is also expected the efficiencies shown inFIGS. 3B and 4A and 4B are achievable in production devices withexisting processes.

In graph 402, tip width 216 is varied from 150 nm to 450 nm, with taperlength 226 being set to 840 nm and tip length 228 being set to 90 nm. InFIG. 5A, a graph 500 shows an estimation of light absorption profile inthe middle of the recording layer. The slot waveguide is tapered downfrom height 202 of 200 nm to height 210 of 40 nm, and gap width 206 of300 nm to gap width 208 of 40 nm. At the 40 nm gap 208, the full widthat half maximum (FWHM) of the optical spot at the medium is 45 nm alongx-direction (cross-track) and 60 nm along z-direction (down-track).Again, this FHWM value is deemed viable for HAMR applications.

For ultrahigh recording density applications, even smaller optical spotsmay be required. In reference now to FIGS. 5B, 6A, 6B, and 7A, graphsshow an optimization for a configuration with a 20 nm gap 208. In thiscase, tip height 210 was set to 20 nm, tip width 216 to 240 nm, and tiplength 228 to 50 nm. In FIG. 5B, graph 502 shows light absorptionefficiency as taper distance 226 is varied from 400 to 1200 nm. In FIG.6A, graph 600 shows light absorption efficiency as a tip length 228 isvaried in ranges between 40 nm and 80 nm, and further wherein tip width216 is varied from 200 nm to 280 nm. In graph 600, taper length 226 isfixed at 650 nm. These results indicate that the NFT can provideacceptable results even given large tolerances in tapering and lappingoperations. While peak efficiencies shown in FIGS. 5B and 6A are lowerthan those shown in 3B and 4B, respectively, these dimensions result insmaller hotspots, as seen in FIGS. 6B and 7A. This may be an acceptabletrade off in many applications.

In FIGS. 6B and 7A, graphs 602 and 700 respectively show profiles ofenergy absorption and temperature rise in the middle of the recordinglayer for a 20-nm gap configuration. For the 20 nm gap plasmon, theoptical spot size becomes 26 nm along x-direction (cross-track) and 39nm along z-direction (down-track). Compared to the 40 nm gap, the 50nm-by-50 nm footprint efficiency drops from 0.035 to 0.022 but the peakabsorption is only slightly reduced. Illumination of 10-mW incidentoptical power raises the peak temperature over 300K at time=2 ns. TheFWHM thermal spot size is 54 nm along x-direction and 57 nm alongz-direction. Therefore, this demonstrates that the design can beoptimized for smaller hotspots, even with a slight reduction in lightabsorption efficiency.

In reference now to FIG. 7B, a diagram illustrates a configuration of anNFT 102A and waveguide 104A according to another example embodiment. Aswas described in relation to FIG. 2A, the channel waveguide 104 includescladding 223 of Al₂O₃. However, in this configuration the core 108A isformed from Ta₂O₅ (n=2.1) instead of TiO_(x). Also, while the metalelements 106 may be formed from the same plasmonic metal (e.g., Au) asshown in FIG. 2A, in this configuration, the gap 103 is filled with adifferent material 702 (SiO₂, n=1.47), and this material 702 alsosurrounds the upper sides of the elements 106. Using a gap material 702of low index of refraction reduces the effective mode index of the slotwaveguide NFT 102A, thereby lowering the index of refraction of thedielectric channel waveguide core 108A required for phase-match betweenthe dielectric waveguide 104A and the slot waveguide 102A for efficientoptical power transfer.

The optical and thermal performance of the configuration was modeledusing a similar analysis as before. For convenience, the same referencenumerals used in describing dimensions of NFT 102 and waveguide 104 inFIG. 2A are also used in the description of analogous dimensions of NFT102A and 104A in FIG. 7B. In the analysis, the core 108A has width 222of 360 nm and height 220 of 300 nm. The slot waveguide 102A is tapereddown from a height 202 of 200 nm to height 210 of 20 nm. The gap istapered from width 206 of 300 nm a width 208 of 20 nm. The spacing 218between the Ta₂O₅ core 108A and slot waveguide 102A is 50 nm. Othersimulation parameters (e.g., related to elements 106 and target media)are the same as previously described.

In FIG. 8A, graph 800 shows optical power absorbed in a 50 nm by 50 nmmedia footprint from the Ta₂O₅ 104A to the slot waveguide NFT 102A as afunction of coupling length 204. For these results 800, the slotwaveguide taper length 226 is 520 nm, tip length 228 is 70 nm, and tipwidth 216 is 200 nm. In FIG. 8B, graph 802 shows optical power absorbedas a function of taper length 226, where coupling length 204 is 1800 nm,tip length 228 is 70 nm, and tip width 216 is 200 nm.

In FIG. 9A, graph 900 shows the optical power absorbed in the recordinglayer as a function of tip length 228, where coupling length 204 is 1800nm, taper length 226 is 650 nm, and tip width 216 is 200 nm. With an 650nm taper length 226 and 1800 nm coupling length 204, maximum efficiencyis seen at tip length 228 of around 70 nm, or about 35 times thecombined taper and coupling length 226, 204. In FIG. 9B, graph 902 showsthe optical power absorbed in the recording layer as a function of tipwidth 216, where coupling length 204 is 1800 nm, taper length 226 is 650nm, and tip length 228 is 70 nm. Graph 1000 of FIG. 10 shows expectedprofile of light absorption in the middle of the recording layer forthis configuration. This shows the energy concentrated well withindesired hotspot with dimensions of less than 50 nm by 50 nm.

It can be seen from these results that slightly better light deliveryefficiency may be obtained using SiO₂ as the gap material 702. This maybe due to lower light absorption in the slot waveguide. The lappingtolerance at 90% efficiency as shown in FIG. 9A is 22 nm, which is 3σ ofcurrent lapping accuracy (σ=6-7 nm). The optical spot size FWHM in themiddle of the recording is 26-nm along X direction and 39 nm along the Zdirection, which is similar to the configurations using Al₂O₃ as the gapmaterial.

As previously described, the NFT and associated components are formedusing layer deposition techniques and other processes associated withsemiconductor wafer fabrication. In the following diagrams, varioustechniques are described that may be used to form any of the embodimentsdescribed herein. A first approach will be referred herein as a“bottom-up” approach, in which case the dielectric channel waveguide isconsidered the “bottom,” and the plasmonic elements 106 forming the NFTare deposited on top. The bottom-up process is generally illustrated inFIGS. 11A-11E and 12A-12E, and in the flowchart of FIG. 16.

In reference now to FIG. 11A, a perspective view of a waveguide core108B on a dielectric layer 1004 shows a beginning step of the bottom upprocess. The core 108B is elongated along a delivery axis 1007 andextends to a media-facing surface 1008 (e.g., ABS). The media facingsurface 1008 may be moved closer to the NFT (e.g., metallic elements1102 shown in FIG. 11C) during later stages of processing, e.g., bytrimming the substrate and layers near to a narrowed tip of the NFT. Thewaveguide core 108B may be formed from any material described herein,such as Ta₂O₅ and TiO_(x). The layer 1004 may be part of the surroundingcladding (e.g., cladding 223 seen in FIG. 2A), and may be formed fromAl₂O₃, or any other suitable material. As seen in FIG. 11B, anotherlayer 1006 of dielectric, e.g., Al₂O₃, is deposited, and thenchemical-mechanical planarization (CMP) is performed to make the waferflat. The CMP stops 50 nm above the surface of the core 108B, whichdefines the core to NFT distance (e.g., dimension 218 seen in FIG. 2A).

In FIG. 11C, a thin layer 1102 of plasmonic material (e.g., Au) isdeposited to tip thickness (e.g., thickness 210 seen in FIG. 2A). Theprocesses shown in FIGS. 11A-11C are also described in blocks 1062 and1604 of FIG. 16. The shape of the layer 1102 may be defined by way oflithographic deposition/etching, and is generally shaped as two metalelements with a gap therebetween joined at a narrow tip proximate themedia surface. In FIG. 11C, the narrow NFT gap dimension (e.g.,dimension 208 seen in FIG. 2A) is not defined, as this feature size maybe difficult to form at this stage using current lithographic processes.Instead, as seen in FIG. 11D, a gap 1104 is etched/formed after definingthe outer shape of layer 1102. After forming the outer shape of layer1102, a number of techniques may be subsequently used to form the gap1104. One of these techniques is shown in FIG. 11E, and another is shownin FIGS. 12A-12D.

A first example of forming the gap 1104 is shown in FIG. 11E, where anangled mill 1106 may be used to form the gap 1104. Using the angled mill1106 involves depositing a layer of photoresist 1108 on top of theplasmonic material 1102, and then cutting the material 1102 through atrench 1110 through the photoresist layer 1108. For example, where witha 100 nm trench opening 1100 in a 150 nm layer of photoresist 1108, a 25degree tilting mill 1106 can be used to obtain a 30 nm gap 1104. Thistechnique is also described in optional portion 1606 of FIG. 16.

Another example method to form the small NFT gap is referred to hereinas “sidewall deposition,” and is illustrated in FIGS. 12A-12D, anddescribed in optional portion 1608 of FIG. 16. As seen in FIG. 12A, amesa 1112 of hard mask material such as amorphous carbon (a-C) is formedon top of layer 1102 so that one side of the mesa 1112 is positionedalong the desired gap location (e.g., gap 1104 seen in FIG. 12D). Then athin layer 1114 (e.g., 30 nm) of alumina (Al₂O₃) is deposited, e.g.,through atomic layer deposition (ALD) on top of the mesa 1112 andplasmonic material 1102. This layer 1114 is conformal so that thecoating on the sidewall of mesa 1112 is the same thickness (e.g., 30 nm)as on the horizontal surfaces. Then a layer 1116 of Cu is deposited overthe alumina layer 1114.

As seen in FIG. 12B, CMP is performed to remove the Cu and alumina layerfrom the top of the a-C mesa 1112. Next, as seen in FIG. 12C, a void1118 is formed by wet-etching the vertical part of layer 1114. Finally,as shown in FIG. 12D, the gap 1104 conforming to the desired dimension(e.g., 30 nm) can be milled using the remaining a-C 1112 and Cu 1116material as a hard mask. Afterwards, the a-C layer 1112 can be ashedaway, and the Cu layer can be wet etched away (not shown).

After formation of the NFT tip as shown in FIG. 11E or FIGS. 12A-12D, athin (e.g., 20-30 nm) a-C stop layer 1202 can be laid over the tipportion of the NFT (e.g., using O₂ ashing) as seen in FIG. 12E. Thisprocess is also described in optional blocks 1610-1613 of FIG. 16. InFIG. 12F, a thick portion 1204 of plasmonic material used for the metalelements the NFT has been formed using a plating or liftoff process. Asseen in FIG. 12G, another layer of alumina 1206 can be deposited aroundthe material 1204. The top surface of alumina 1206 and the NFT material1204 can be planed using CMP. Afterwards, sloped wall 1208 can be formedusing reactive ion beam etch (RIBE) milling stopping at the a-C stoplayer 1202. Finally, as seen in FIG. 12H, the a-C layer 1202 has beenashed away to show the final structure of the NFT metal elements (e.g.,elements 106 seen in FIG. 1).

As mentioned above, a second process may be used to form the NFTembodiments described herein. This second process, which is shown by wayof example in FIGS. 13A-13F, is referred to as the “upside down”approach. This approach is also shown in the flowchart of FIG. 17. Asseen in FIG. 13A, a thin (e.g., 10 nm) seed layer 1302 of plasmonicmaterial (e.g., Au) and a thin (e.g., 20 nm) a-C stopping layer 1304 aredeposited on a substrate (not shown). These 1302, 1304 layers can bepatterned by etching, and a relatively thick (e.g., 220 nm) layer 1306of alumina is deposited thereafter. As seen in FIG. 13B, a 20 degreeslope trench 1308 is RIBE milled through layer 1306, stopping on the a-Clayer 1304.

In reference now to FIG. 13C, a portion of the a-C layer 1304 exposed inthe trench 1308 is ashed away, leaving the plasmonic seed layer 1302exposed. Then, a photoresist layer 1310 may be applied, which will beused to pattern the thick part 1312 of the NFT metal elements (see FIG.13E), after which the photoresist 1310 is removed. Alternatively, thethick part 1312 of the NFT could be created using a liftoff process, asrepresented by FIG. 13D. In reference now to FIG. 13E, the seed layer1302 may be slightly milled away after removal of photoresist (if used),and alumina is deposited to fill the voids, e.g., in gap portion 1314.This can then be CMP processed to plane the top surface and set thedesired thickness of plasmonic elements 1312. On top of this surface, asseen in FIG. 13F, a thin (e.g., 30 nm) layer 1316 of plasmonic materialis deposited to form the thin portion of the NFT with tip shape 112. Atthis point, the thin layer 1316 can be overlaid with dielectric andwaveguide portions (not shown) in an arrangement similar as that shownin FIG. 11A-11D and a media-facing surface formed, e.g., by cutting theassembly near the narrow tip of the NFT.

Because the NFT described hereinabove may be part of a HAMR writingapparatus, a recording pole of a read/write head may be located in closeproximity with the NFT. An example of how a recording pole 1502 may bepositioned according to one example embodiment is shown in the crosssectional view of FIG. 15A. As in previous illustrations (e.g., FIGS. 1and 2B), the waveguide core 108 may extend to at or near the writingsurface (e.g., ABS) 114. In such a case, the recording pole 1502 may beplaced on a side of the metal elements 106 opposite from the waveguidecore 108. In an alternate configuration seen in FIG. 15B, a shortenedwaveguide core 108C may be disposed between the metal elements 106 and arecording pole 1504. In such a case, a portion of the recording pole1504 proximate the ABS 114 may lie between the end of waveguide core108C and the ABS 114.

The foregoing description of the example embodiments has been presentedfor the purposes of illustration and description. It is not intended tobe exhaustive or to limit the invention to the precise form disclosed.Many modifications and variations are possible in light of the aboveteaching. Any or all features of the disclosed embodiments can beapplied individually or in any combination are not meant to be limiting,but purely illustrative. It is intended that the scope of the inventionbe limited not with this detailed description, but rather determined bythe claims appended hereto.

1. An apparatus comprising: a plasmonic transducer that includes atleast two metal elements with a gap therebetween, wherein the metalelements are elongated along a plasmon-enhanced, near-field radiationdelivery axis, and wherein cross sections of the metal elements in aplane normal to the delivery axis vary in shape along the delivery axis;and a waveguide disposed along an elongated side of the plasmonictransducer, wherein the waveguide is optically coupled to the plasmonictransducer along the elongated side.
 2. The apparatus of claim 1 whereinthe cross sections become narrower in a down track direction at anoutput end of the plasmonic transducer.
 3. The apparatus of claim 2,wherein the gap becomes narrower in a cross track direction at theoutput end of the plasmonic transducer.
 4. The apparatus of claim 3,wherein each of the metal elements comprise a tip of constantcross-sectional shape along the delivery axis separated by the narrowedgap at the output end of the plasmonic transducer.
 5. The apparatus ofclaim 1, wherein the metal elements each comprise: a tip portionproximate an output end of the plasmonic transducer; a taper portioncoupled to the tip portion and increasing in cross sectional area awayfrom the output end; and a coupling portion coupled to the taper portionand disposed along the waveguide to receive energy from the waveguide.6. The apparatus of claim 5, wherein the coupling portion has a constantcross sectional area along the delivery axis.
 7. The apparatus of claim1, wherein the waveguide extends to an output end of the plasmonictransducer.
 8. The apparatus of claim 1, wherein the metal elements areco-planar on a substrate-parallel plane.
 9. An apparatus comprising: aplasmonic transducer that includes at least two metal elements with agap therebetween, wherein the metal elements are elongated along aplasmon-enhanced, near-field radiation delivery axis and each comprise:a tip portion proximate an output end of the plasmonic transducer andhaving a first cross sectional area relative to a plane normal to thedelivery axis; a coupling portion at an input end of the plasmonictransducer and having a second cross sectional area relative to theplane that is greater than the first cross sectional area; and a taperportion coupled between the tip portion and the coupling portion,wherein the taper portion varies from the first to the second crosssectional area along the delivery axis.
 10. The apparatus of claim 9,further comprising a channel waveguide disposed along the delivery axisproximate at least the elongated coupling portions of the metalelements.
 11. The apparatus of claim 10, further comprising a dielectriclayer disposed between a core of the waveguide and the metal elements.12. The apparatus of claim 9, wherein the gap is 40 nm or less at theoutput end, and wherein the tip portions has a side dimension proximatethe gap of 40 nm or less.
 13. The apparatus of claim 9, wherein acombined length of the coupling and taper portions along the deliveryaxis is at least 30 times a length of the tip portion along the deliveryaxis.
 14. The apparatus of claim 9, wherein the metal elements areco-planar on a substrate-parallel plane.
 15. A method comprising:forming, on a substrate, a waveguide having an elongated delivery axisdelivery that extends to a media-facing surface; forming at least twometal elements with a first gap therebetween over the waveguide, whereinthe first gap is elongated along the delivery axis, and wherein the atleast two metal elements are joined at a narrowed tip proximate themedia surface; and forming a small gap through the narrowed tip, thesmall gap extending from the gap to the media-facing surface.
 16. Themethod of claim 15, wherein forming the small gap comprises: depositinga layer of photoresist on the narrowed tip; forming a trench through thephotoresist layer; and cutting the small gap through the trench via anangled mill.
 17. The method of claim 15, wherein forming the small gapcomprises: depositing a first hard mask layer on part of the narrowedtip so that an edge of the hard stop layer is along the small gap;depositing a conformal coating over at least the edge of the first hardmask layer, the conformal coating having a thickness conforming to adimension of the small gap; depositing a second hard mask layer over theconformal coating; planing the second hard mask layer to expose theconformal coating over the edge of the first hard mask layer between thefirst and second hard masks; and etching the exposed conformal coatingbetween the first and second hard mask to form the small gap.
 18. Amethod comprising: forming a thin stop layer over a metallic seed layerdisposed on a substrate; depositing a thick layer of dielectric materialover the thin stop layer; milling a trench having an angled wall in thethick layer; removing a portion of the thin stop layer within the trenchto expose the metallic seed layer; and forming at least two metalelements with a gap therebetween in the trench, wherein the gap iselongated along a delivery axis of a waveguide that extends to a mediafacing surface, wherein the at least two metal elements are angled toform a narrowed end proximate a media-facing surface, and wherein thenarrowed end is further narrowed by the angled wall of the trench. 19.The method of claim 18, wherein forming the at least two metal elementscomprises: applying a photoresist layer in the trench, wherein thephotoresist layer comprises voids in the shape of the at least two metalelements; filling in the voids to form the at least two metal elements;and removing the photoresist material.
 20. The method of claim 18,further comprising: filling the gap with a second dielectric material;planing the metal elements and second dielectric material; andoverlaying a thin layer of metallic material over the metal elements toform a narrowed tip at the narrowed end, wherein the narrowed tipcomprises a small gap extending from the media-facing surface to the gapof the at least two metal elements.